1. Field of the Invention
The invention relates to a method for forming an isolation, and more particularly to reduce a cap size in a trench isolation of an aspect ratio greater than a predetermined value.
2. Description of the Related Art
FIGS. 1a to 1c are cross sections of the conventional method for forming a trench isolation.
In FIG. 1a, a silicon substrate 101 is thermally oxidized to form a pad oxide layer 102, and a silicon nitride layer 103 is doped on the pad oxide layer 102 by chemical vapor deposition. A photoresist layer 104 is coated on the silicon nitride layer 103, and a pattern is defined by photolithography to expose an area for forming isolation. The silicon nitride layer 103 and the pad oxide layer 102 are sequentially etched using the photoresist layer 104 as a mask.
In FIG. 1b, the semiconductor substrate 101 is etched to form a trench 105 for defining an active region using the silicon nitride layer 103 and the pad oxide layer 102 as masks after the photoresist layer 104 has been removed. The trench 105 is thermally oxidized to form a thin silicon oxide layer to act as a lining oxide layer 107 on a bottom and a sidewall of the trench 105.
An oxide layer 108 is formed on the silicon nitride layer 103, and the trench 105 is filled with the oxide layer 108. The oxide layer 108 in the trench 105 has a cap 106 because the oxide layer 108 is difficult to fill into a small trench 105 particularly when the aspect ratio of the trench 105 is greater than 6.
In FIG. 1c, chemical mechanical polishing is performed on the oxide layer 108 to remove the excess silicon nitride layer 103 forming the isolation 108a. 
In FIG. 1d, the silicon nitride layer 103 and the pad oxide layer 102 are sequentially removed to leave the isolation 108a. 
The material employed to form the isolation 108a is similar to the pad oxide layer 102, and the isolation 108a is etched when the pad oxide layer 102 is removed by wet etching. The cap 106 is exposed, and the etching solution fills the cap 106. The size of the cap 106 increases because the isolation 108a is etched, thus the reliability of the element is reduced.